Shaft packings are required to provide sealing of the turbine rotor or shaft between the turbine shells or the exhaust hood and the atmosphere. During normal turbine operations, the endpackings can be divided into two distinct groups, pressure packings and vacuum packings. Pressure packings generally prevent steam from blowing out into the turbine room. Vacuum packings generally seal against the leakage of air into the condenser. Known steam seal systems largely address these issues by utilizing the steam leaking from the pressure packings to help seal the vacuum packings.
Current steam seal systems generally have a single set point sub-optimized design. For example, these designs may provide an unfired guarantee loading with a self-sealing load point (“SSLP”) of about seventy percent (70%). When a steam turbine “self seals”, the terms generally refer to the condition where the pressure packing seal steam flow is sufficient to pressurize and seal the vacuum packings. In higher load conditions such as a supplementary firing, however, the pressure packing steam flow going to the steam seal header increases but the vacuum packing requirement may not vary such that the SSLP may be as low as about thirty percent (30%). The additional steam coming from the pressure packings into the steam seal system thus may be dumped to the condenser without extracting any work. Similarly during low load operations, the pressure packing steam flow may be reduced significantly from the design point, but the vacuum packing steam flow requirements again may not vary. In such a situation, the steam seal system may not be sufficient and an extra flow may be required from the throttle steam at a significant loss in performance.
There is a desire therefore for an improved steam seal system so as to maintain a substantially consistent self-sealing load point across numerous loading situations. Such a constant self-sealing load point should improve overall power output and provide heat rate improvement.